1. Field of the Invention
The present invention pertains to hydraulic slide valves of the type used for reinforcing props in mines.
2. Description of the Prior Art
The reinforcing pilings used in mines include push-cylinders which are squeezed between the top bed and the stope or gallery walls. While the push-cylinders apply a considerable thrust (usually 90 or 100 tons each), they must sustain the pressure applied by the adjacent beds. Since the latter pressure may increase and reach a virtually unlimited value, it is necessary to equip the hydraulic system of the reinforcing cylinders with a pressure regulating valve, also called a slide valve.
In order to operate satisfactorily, the slide valve should perform two functions. First, the slide valve should open to allow for an outflow of fluid as soon as pressure rises above a preset level. The valve should open immediately at the preset level in order to permit a flow of liquid, which may become significant, and avoid irreversible damage to the piling including that which may occur when the adjacent beds suddenly dry out following what is known as a "top bed blow". Secondly, the slide valve should close immediately when the liquid pressure falls below a preset level, so as not to impair the prop as a means of reinforcement.
The slide valves that are presently used offer three major disadvantages. Some of these lack durability. Others include wear components whose condition cannot be checked from outside and deprive the prop of a safety device which would prevent irreversible damage caused by the eventual motion of the adjacent beds. Finally, other types of valves close as the pressure reaches a preset level, which is already too low with respect to the pressure which controls their opening.